Wednesday, January 18, 2006

Resurrection of Winter (for the USA)?

The emerging La-Nina (with SST anomalies -1.0C and lower along the equator from 170W to near South America) has already impacted the circulation state of the atmosphere. This includes persistent tropical convective forcing centered just northwest of Australia, relative AAM about 2 standard deviations below the 1968-1997 climatology, and zonal mean westerly flow shifted poleward of normal. Additionally, bitterly cold Arctic air has been expanding not only across much of Asia, but also Alaska and northwest Canada. This base state circulation is represented by SDM Stage 1 (see previous posts for link), and these initial conditions are relevant for any upcoming predictions through at least week 2 (February 1).

In addition to other forcing-response processes discussed in previous writings, the recent Sudden Stratospheric Warming (SSW) 0f at least 30 deg C must also be given consideration. In summary, SSWs disrupt the circulation of the winter Stratospheric polar vortex by producing easterly wind anomalies. These easterly anomalies can and do propagate into the Arctic troposphere, and contribute to rising heights/pressures. The latter can lead to a projection onto the negative phase of the Arctic Oscillation (AO). The recent USA Hazards Assessment issued by CPC also discusses this concern http://www.cpc.ncep.noaa.gov/products/predictions/threats/threats.htm, and additional information on the AO can be found at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml

With the EA jet having retracted, our thought is the north Pacific storm track will be directed to the west of the Aleutians, allowing for a ridge to amplify initially across the central and east Pacific by early-middle of next week. Storms would continue to impact the Pacific Northwest. Considering the current location of the tropical convection (centered ~10S/135E), there is concern the atmosphere may transition into SDM Stage 2 by roughly next weekend. That suggests a possibility of an east Pacific ridge to link up with positive height anomalies all across the Arctic (which would extend into the North Atlantic; i.e., negative phase of the NAO), and allow for the delivery of Arctic air particularly into the central USA (centered on the northern Rockies and Plains). While the west coast would become dry, the weather may be quite active across at least the eastern two-thirds of the country. Uncertainty remains very high with this scenario. However, many models are starting to pick up on this notion, particularly the GFS ensemble and ECMWF deterministic run (1200 UTC 18 January initial conditions).

For southwest Kansas, if winter (at least in terms of well below normal temperatures) is to return, that may not occur until later next week. In the meantime, after one weak storm goes by this Friday, another rain/snow event is possible on about Sunday-Monday. Perhaps a decent precipitation situation (still low optimism) may be possible around Wednesday-Thursday of next week (25-26 January). It may be after this storm that the circulation transitions to SDM Stage 2, meaning the possibility of Arctic air spreading across the Plains.